There have been proposed various cathodes of low hydrogen overvoltage, in particular, a cathode for electrolysis of alkali metal halide aqueous solution. Of these electrodes, the one which has already been proposed by the present applicant and is disclosed in the unexamined Japanese patent publication No. 112785/1979 possesses a remarkable effect relative to its low hydrogen overvoltage and durability in comparison with that of electrodes which has thus far been known. However, as the result of further studies having been made, the present inventors have found that, depending on circumstances, even the electrode as disclosed in the above-mentioned unexamined Japanese patent publication does not always exhibit sufficient durability, and, after strenuous efforts having been made for the solution of this problem, they have accomplished the present invention.
Production, in an electrolytic cell of alkali metal halide aqueous solution, of halogen gas from its anode compartment, and of aqueous solution of caustic alkali and hydrogen gas from its cathode compartment is the industrialized method of manufacturing chlorine and caustic alkali, which has already been well known. As the cathode for this electrolytic cell, a cathode of a low hydrogen overvoltage as mentioned above is used preferably. However, the above-mentioned electrolytic cell is liable to bring about stoppage in operation in the course of its running for various reasons, and, in this case, the hydrogen overvoltage is recognized to increase when its operation is resumed. As the result of pursuing this phenomenon in depth, the present inventors discovered that, in the case of stoppage in operation of the electrolytic cell by a method, in which the anode and the cathode are short-circuited through a bus bar, the cathode is oxidized by reverse current to be generated at the time of the short-circuiting, and that, in the case of the cathode being made up of nickel and cobalt as its active components, these substances become modified to hydroxides to thereby decrease the electrode activity, which does not return to the original active state even after its operation has been resumed (i.e., increase in the hydrogen overvoltage).
Moreover, it has been found out that, even in the method of stopping operation in the electrolytic cell by ceasing conduction of electric current without short-circuiting the anode and the cathode, if the cathode is immersed over a long period of time in aqueous solution of NaOH at a high temperature and with a high concentration, the active component of the cathode, when it is made of nickel or cobalt, rushes into a corrosion potential to be modified into its hydroxide (this reaction is also a sort of electro-chemical oxidation reaction), whereby the electrode activity lowers.